Refining of steam-cracked gasolines with molten salt



Apnl 14, 1964 P. VISSER ETAL 3,129,165

REFINING OF STEAMCRACKED GASOLINES WITH MOLTEN SALT Filed March 22, 1961 PRODUCT SEPARATION CHAMBER COMBUSTION PRODUCTS $TEAM STEAM REGENERATION CHAMBER REACTION 2 CHAMBER REGENERATION REGENERATION GAS GAS' OPTIONAL STEAM FEED INVENTORSI PIETER VISSER GERRIT VAN DER MOST SYDNEY MULLER THElR AGENT United States Patent 3,129,165 REFINING 0F STEAM-CRACKED GASOLINES WITH MOLTEN SALT Pieter Visser and Gerrit van der Most, Amsterdam, and Sydney Muller, The Hague, Netherlands, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Mar. 22, 1961, Ser. No. 97,564 Claims priority, application Great Britain May 9, 1960 4 Claims. (Cl. 208-483) This invention relates to improvements in or relating to a process for the refining of hydrocarbon oils, particularly to a process for the refining of steam-crackedhydrocarbon oils.

In the present specification the term steam-cracked hydrocarbon oils denotes normally liquid hydrocarbon oils and fractions thereof that are obtained as product in the thermal cracking of hydrocarbon oils in the presence of steam.

The thermal cracking operations in which these steamcracked hydrocarbon oils are obtained as a product are carried out by subjecting a hydrocarbon oil in admixture with steam to temperatures between approximately 550 C. and 900 C. and preferably between 750 C. and 800 C. The pressure is preferably below approximately 5 atm. abs. The quantity of steam employed is usually 01-10 parts by weight and preferably about '1' part by weight per part by weight of starting material. Cracking of hydrocarbon oils at these high temperatures in the presence of steam is mainly applied for the preparation of lower alkenes, in particular ethylene and propylene, required as base materials in the chemical industry. In such steam-cracking treatment, which is preferably carried out in a tubular reactor, usually more than 50% by weight of the starting hydrocarbon oil is converted into compounds having four or fewer carbon atoms in the molecule.

The hydrocarbon oils that are obtained as -a product (steam-cracked hydrocarbons), are of a very special type in that on the one hand they are extremely unstable owing to the presence of a relatively high content of very reactive unsaturated hydrocarbons, in particular diolefins and alkynes, and on the other hand they contain particularly valuable aromatic compounds in high proportions.

This is particularly true for the gasoline fractions of the steam-cracked hydrocarbon oils (which are hereinafter referred to as steam-cracked gasolines). which in addition to intolerably high proportions of unstable compounds such as diolefins and alkynes, contain very desirable, and in themselves stable, motor gasoline components in the form of a high percentage of aromatic compounds. In fact, the aromatics content of steam-cracked gasolines is usually more than 30% by weight and often more than 60% by weight. In most cases these gasolines are practically free of sulfur, i.e. they have a content of sulfur compounds (calculated as elemental sulfur) which is considerably lower than 011% by weight.

A number of methods have already been used or proposed for the refining of steam-cracked gasolines, including one-stage and multi-stage hydrogenative refining or treatment with phosphoric acid. However, certain drawbacks are inherent to these previous methods. Thus, the hydrogenation processes have the drawback of requiring substantial amounts of hydrogen which is usually in very short supply at refineries, whereas the refining with phosphoric acid entails waste disposal problems.

It is an object of the invention to provide a process for improving the properties of steam-cracked gasolines. It is a particular object of the invention to provide a process for improving the octane number and reducing the olefin and diolefin (non-aromatic) content of steam-cracked gasolines. Other objects will become apparent from the 3,129,165 Patented Apr. 14, 1964 further description of the invention, wherein reference will be made to the figure showing an apparatus suitable for carrying out the process.

It has now been found that steam-cracked hydrocarbon oils may be refined in a very simple manner to produce valuable products of greatly increased stability by contacting them with molten salts at elevated temperatures.

Thus, the invention relates to a process for the refining of steam-cracked hydrocarbon oils, which comprises contacting the material to be refined with a molten salt or a mixture of molten salts at elevated temperature.

The main products formed during this contacting op eration are a normally liquid fraction of greatly increased stability that has quite a high concentration of aromatic hydrocarbons and a normally gaseous fraction that has a surprisingly high ethylene content. Moreover, a small amount of heavy carbonaceous material is formed,-which is retained by the molten salt and may be removed therefrom by regeneration of the molten salt(s) e.g. by treating with an oxidizing gas, such as air, steam, carbon dioxide or combustion gases.

The preferred starting materials are steam-cracked gasolines with an initial boiling point above about 35 C. and a final boiling point not exceeding 250 C., as wellas fractions thereof.

When starting with steam-cracked gasolines or fractions thereof the normally liquid reaction product is a highly aromatic material of very satisfactory stability, due'to the substantial absence of diolefins, alkynes, mono-olefins and other unstable components. For this reason, this normally liquid reaction product, or at least that part thereof that boils in the gasoline range, is very suitable as a premium gasoline component. Moreover, this liquid reaction product is a very attractive source for obtaining in a simple way individual aromatics of high purity. Due to the substantial absence of non-aromatic components in the same boiling range a simple fractionation is sufii'cient for this purpose. In fact it was found that the benzene fraction thus obtained required only a mild treatment with /a% by weight of concentrated sulfuric acid for conversion into a nitrogen grade" benzene. It should be observed that recovery of substantially pure aromatic-s from unrefined steam-cracked gasolines involves additional azeotropic or extractive distillation, which operation is not required if the crude steam-cracked gasolines are first refined according to the present invention.

The reaction temperature is generally in the range of 700 C. to 1200 C., temperatures in the range of 800 to 1000 C. being preferred. The contact time is usually from about 0.005 second to about 0.1 second and times in the range of 0.01 to 0.05 second are generally preferred. Temperature and contact time are more or less reciprocal in that longer contact times are required at lower reaction temperatures and shorter contact times are sufficient at higher reaction temperatures in order to attain a particular degree of conversion. Increasing the contact time at a given temperature results in higher conversions, higher purities of the benzene fraction and lower toluene yields. At very high contact times increased yields of polynuclear aromatics are observed.

The pressure may be sub-atmospheric, atmospheric or super-atmospheric.

The steam-cracked hydrocarbon oils may be contacted as such-with the molten salt(s), but it is sometimes ad'- vantageous that the contacting be carried out in the presence of steam and/or hydrogen (or if desired in the presence of a hydrogen-containing gas, such as surplus gas from a catalytic reforming unit or the hydrogen'and methane containing tail gases from a cracking unit) and/ or other reactive materials.

The addition of steam and/or hydrogen results in-increased yields of useful products; moreover hydrogen, particularly at higher pressure appears to cause a shift in the product distribution in the sense that lower boiling aromatics rather than higher boiling ones are obtained.

The steam-cracked hydrocarbon oil may be contacted with the molten salt(s) in any convenient way. Thus, the oil to be treated may be passed in a finely divided state through a bath of molten salt(s), from the surface of which the reaction products escape. It is preferred, however, to pass the material to be treated, together with the molten salt(s), through a reaction zone, with subsequent separation of the salt(s) from the reaction mixture. This modification is preferred because it enables the molten salt(s) to be regenerated in an easier and more complete manner. In fact, it was found that the heavy carbonaceous material, referred to above, which is formed in small quantities as a by-product and is retained by the molten salt(s), is rapidly transformed into free carbon when left in contact with the molten salt(s), and that this free carbon is very difficult to eliminate by combustion in contradistinction to the heavy carbonaceous materials themselves which can be combusted fairly easily. For this reason it is preferred that the contact time between the molten salt(s) and the hydrocarbons be rather short and that the molten salt(s) are regenerated as rapidly as possible after their separation from the vaporous reaction mixture. This explains the preference for the contacting operation just mentioned, in which the feedstock is passed, together with the molten salt(s), through a reaction zone, with subsequent separation of the salt(s) from the reaction mixture, because this allows both a short contact time and a rapid regeneration.

The regeneration is usually carried out by combustion of the heavy carbonaceous material with an oxidizing gas. The oxidizing gas may be air or air enriched with oxygen, which oxidizes the carbonaceous constituents absorbed by the molten salt; the heat thus liberated during the regeneration serves to reheat the salt. Other gases with oxidizing properties such as steam, carbon dioxide and combustion gases may also be used.

It is, of course, possible that the quantity of heat thus transmitted to the salt is not sufiicient for it to reach the temperature nedeed for the refining process. This deficiency may be overcome by introducing into the regeneration zone, in addition to the oxidizing gas, a combustible gas or liquid, of which the amount is so regulated as to transmit a quantity of heat sufiicient to raise the temperature of the salt to the value required for the refining process. It is, however, also possible to achieve this result by passing into the regeneration zone the hot combustion gases supplied from a combustion unit outside the plant, regulating the amount of these gases so as to transmit a sufiicient quantity of heat to the salt. By controlling the above-mentioned amounts maximum efiiciency can be attained while none of the heat transmitted to the salt need be lost, for example by radiation, as it passes from the regeneration zone into the reaction chamber, if, as in the preferred process, these zones are in direct communication with each other.

After the separation of the molten salt(s) from the vaporous reaction mixture the latter is generally still unstable at the prevailing high temperatures. For this reason this mixture should preferably be quenched to a sufficiently low temperature, e.g. by injection of steam and/or water. The quenching device is preferably arranged as close as possible to and directly behind the point where the salt and the gaseous reaction products are actually separated, so that the reaction products have already been quenched when they pass through that part of the separation zone on the walls of which no salt has separated out. This prevents solid matter, e.g. carbon, from being deposited on the part of the wall in question, which might lead to operational disturbances.

In a particularly preferred embodiment of the invention the refining is carried out by passing the steam-cracked hydrocarbon oil in admixture with entrained molten salt at high speed through a tubular reaction chamber, separating the vaporous reaction products from the salt in a separation zone; discharging the reaction products from the separation zone and passing the salt leaving this zone via a liquid seal into a regeneration zone surrounding the reaction zone, in which regeneration zone the salt is regenerated by the action of an oxidizing gas and also heated to the required refining temperature, the salt being finally returned into the tubular reaction chamber.

This technique of passing the base material together with the salt through a tubular reaction chamber at high velocity and separating the salt from it immediately afterwards is very attractive, not only because this type of operation allows a very close control of the contact time, but also because the deviations from the average contact time are very small. As a consequence optimal results are obtained with this particular contacting technique.

In the particularly preferred embodiment just mentioned the separation zone is adjacent to the regeneration zone, which surrounds the reaction chamber. In view of this, a liquid seal is disposed between the two zones, which prevents on the one hand gaseous reaction products from penetrating into the regeneration zone and on the other, the regeneration medium or combustion products from mixing with the gaseous reaction products. In order to ensure very rapid regeneration after the reaction the return velocity of the separated salt from the separation zone to the regeneration zone is preferably regulated so as to give the salt a residence time of at most 0.5 second in the separation zone. This result is easy to obtain by giving appropriate dimensions to the space forming the separation zone and to its discharge openings. The lower the reaction temperature the longer should be the residence time of the salt in the separation zone, as this allows the greatest possible amount of gaseous reaction products to separate from the salt. In practice, the residence time in the separation zone may be much shorter than the value of 0.5 second and may, for example, be 0.1 second or even less, down to eg 0.05 second.

As regards the salts to be used in this process, it should be noted that they should have both a satisfactory thermal conductivity and a low vapor pressure at the prevailing operating temperatures, in order to prevent as far as possible entrainment of the salt by the vaporous reaction products. Moreover, the salts should preferably be such as not to attack the construction materials of the apparatus. Mixtures of salts may in many cases be usefully employed; In the case of binary or multiple mixtures the proportions of the components should be preferably so chosen as to make them correspond to the eutectic point.

Some of the heat transfer salts suited to this purpose are of the kind often designated as hardening salts in the metallurgical industry. Other salts which give satisfactory results are, for instance, alkali metal sulfates, chlorides or their double salts. A potassium salt or a mixture containing a potassium salt may be advantageous when free carbon is likely to form, since carbon is somewhat less difficult to remove by combustion when present in admixture'with potassium salts than with other salts. Other metal chlorides, metal sulfides or cyanides may also be used as heat transfer media in the process. Specific materials include lithium chloride, cadmium chloride, lead chloride, sodium cyanide, sodium aluminum chloride, calcium fluoride, antimony sulfide and their admixtures.

'Ihe invention will now be described with reference to the accompanying drawing.

Referring to this drawing, a vertical tubular reaction chamber 1 is disposed within a cylindrical chamber 2 the lower portion of which constitutes a regeneration chamber. A separation chamber 3 is disposed at the end of reaction chamber 1 and partly surrounds it. Reaction chamber 1 has inlet openings 4 through which molten salt may flow and which communicate directly with the regeneration chamber. On the discharge side, the reaction chamber communicates directly with the separation chamber 3 through an opening 5. The separation chamber contains an inverted basin 6 which prevents the molten salt entrained in the gaseous reaction products from escaping with said products via a product discharge line 7. The separation chamber 3 is also provided with a discharge opening 8 through which molten salt may flow into the regeneration chamber. A feed line 9 for the steam-cracked hydrocarbon oil communicates via a valve 10 with a feed inlet nozzle 13 axially situated in the reaction chamber 1 in the immediate vicinity of the openings 4. If desired, steam and/or hydrogen and/or other components may be added to the system via a line 11, fitted with a valve 12. It should be noted that the steam-cracked hydrocarbon oil is preferably supplied to the system in the liquid state and at ambient temperature through line 9 and that there is substantially immediate vaporization of the feed material when this material comes into contact with the very hot molten salt at the outlet of nozzle 13. A supply line 14 for oxidizing regeneration gas communicates with a number of annular lines 15 arranged in the regeneration chamber, and which are provided with a number of openings.

The regeneration chamber also comprises an outlet 16 for discharging combustion products from the regeneration process.

While the apparatus is in operation, the oil to be refined is ejected into the reaction chamber through the nozzle 13, whereby molten salt is entrained with the oil to be refined and intimately mixed therewith. The amount of entrained salt may be varied, for example, by varying the size of opening 4 by means of an adjustable cylindrical slide 17.

The discharge opening 8 for the salt from the separation chamber is an annular slit between the cylindrical part of the chamber 3 and the circular base plate (18. By vertically moving the plate 18 in relation to the cylindrical wall of chamber 3, the size of the opening may be varied, thereby regulating the quantity of salt passing to the regeneration chamber. The position of the cylindrical wall of the chamber 3 may be regulated at the point 19 in relation to the rest of the apparatus.

Finally, the separation chamber 3 is provided with a device 20 for quenching gaseous reaction products by injection of steam.

The reaction mixture that leaves the reaction chamber through outlet 7 is quenched with steam and then with water and is further processed in a conventional manner for the recovery of useful products.

The invention will be further illustrated by the following example.

Example The starting material was a fraction of a steam-cracked gasoline, which fraction showed the following properties:

Initial boiling point (ASTM) C 40 Final boiling point (ASTM) C i163 Maleic acid/anhydride value 110 F-l octane number (unleaded) 94 and had the following approximate composition (in percent by weight):

Benzene 24 Toluene 12 Saturated compounds 43 Mono-olefins 10 Di-olefins l 1 From the high maleic acid anhydride value and the high value for the (ll-olefin content it is obvious that this Run B Temperature of molten salt at reactor inlet, C 882 Protgct Yields (percent by weight on intake):

oii. Cell. Conn can;

Fraction caught in a cold trap (kept at 0 Liquid fraction with upper cutting point, 73 0. Fraction boiling between 73 and C Fraction boiling between 150225 O Fraction boiling above 225 C. (by difference) HM NH s s s s s s ps s ps z- Analysis liquid fractious (percent by Weight intake):

Fraction 73l50 C.-

Benzene Tolucne p. and m. Xylene Fraction 73150 O.-

o. Xylene Styrene Fraction l50225 Napthalene Other components (mainly styrene-like compounds) The fraction boiling from 75 to 150 C. obtained in run A had the following properties:

Maleic acid anhydride value l0 F-l octane number (unleaded) (Wiese-scale)--- 117.7

These figures show that this fraction has a greatly increased stability (low maleic acid anhydride) as well as a considerably increased octane number; hence it is an eminently suitable component for premium gasoline.

From a comparison between runs A and B it appears that naphthalene production is somewhat higher in run B than it is in run A. This is caused by the more severe reaction conditions (higher temperatures) applied in run B. Other experiments revealed that significant naphthalene yields can be realized by increasing the reaction temperatures and/ or the contact time to a sufficiently high extent. This increase in naphthalene yield appears to be accompanied by a decrease in the yields of benzene and toluene.

We claim as our invention:

1. In the process for the conversion of steam-cracked gasoline, wherein said gasoline comprises substantial proportions of aromatics, olefins and diolefin hydrocarbons in the gasoline boiling range, the steps comprising subjecting said steam-cracked gasoline to a reaction temperature of 7001200 C. in contact with a molten salt wherein the cation thereof is selected from the group consisting of lithium, sodium, potassium, calcium, barium, cadmium, lead, antimony, and sodium aluminum; and wherein the anion is selected from the group consisting of sulfate, chloride, fluoride, sulfide, and cyanide, for a contact time of 0.0050.1 second, separating the molten salt from the conversion product, said product comprising substantial reduction in non-aromatic hydrocarbons in the normally liquid boiling range and substantial formation of normally gaseous hydrocarbons.

2. A process for the treatment of steam-cracked gasoline to enhance the quality thereof which comprises contacting the gasoline with a molten salt wherein the cation thereof is selected from the group consisting of lithium, sodium, potassium, calcium, berium, cadmium, lead, antimony, and sodium aluminum; and wherein the anion is selected from the group consisting of sulfate, chloride, fluoride, sulfide, and cyanide, at a temperature of 700- 1200 C. for a period of 0005-01 second to effect reactions converting non-aromatic steam-cracked gasoline components into normally gaseous hydrocarbons, separating the salt from the hydrocarbon reaction product and separating normally gaseous hydrocarbons from normally liquid hydrocarbons having a substantially increased aromatic content and higher octane number than those of the steam-cracked gasoline.

3. A process for treating a steam-cracked gasoline boiling within the temperature range of about 35 C. and about 250 C. to improve the octane rating and reduce the non-aromatic content thereof which comprises contacting said gasoline at temperatures of about 800- 1000 C. with a molten salt wherein the cation thereof is selected from the group consisting of lithium, sodium,

potassium, calcium, barium, cadmium, lead, antimony, and sodium aluminum; and wherein the anion is selected from the group consisting of sulfate, chloride, fluoride, sulfide, and cyanide, for 0.010.05 second, separating the molten salt and separately recovering a normally gaseous hydrocarbon product and a normally liquid gasoline product having improved octane number and reduced non-aromatic hydrocarbon content.

0 4. The process for the production of a gasoline which comprises steam-cracking a hydrocarbon oil by heating said oil at a temperature between about 550 and about 900 C. in the presence of 01-10 parts by weight of steam per part of oil, isolating from the cracked product a steam-cracked gasoline boiling within the range from about 35 C. to about 250 C., contacting said gasoline with a molten salt wherein the cation thereof is selected from the group consisting of lithium, sodium, potassium, calcium, barium, cadmium, lead, antimony, and sodium aluminum; and wherein the anion is selected from the group consisting of sulfate, chloride, fluoride, sulfide, and cyanide, at a temperature of about 7001200 C. for 0005-01 second and isolating from the product so obtained a gasoline having relatively high octane number and high aromatic content.

References Cited in the file of this patent UNITED STATES PATENTS 1,620,075 Clancy Mar. 8, 1927 1,937,914 Pines Dec. 5, 1933 2,108,438 Kimball Feb. 15, 1938 2,245,157 Pier et al. June 10, 1941 2,416,023 Schulze et al. Feb. 18, 1947 2,749,288 Watkins June 5, 1956 2,768,935 Watkins Oct. 30, 1956 2,914,459 Mills et al. Nov. 24, 1959 3,051,645 Wilson et al. Aug. 28, 1962 

1. IN THE PROCESS FOR THE CONVERSION OF STEAM-CRACKED GASOLINE, WHEREIN SAID GASOLINE COMPRISES SUBSTANTIAL PROPORTIONS OF AROMATICS, OLEFINS AND DIOLEFIN HYDROCARBONS IN THE GASOLINE BOILING RANGE, THE STEPS COMPRISING SUBJECTING SAID STEAM-CRACKED GASOLINE TO A REACTION TEMPERATURE OF 700-1200*C. IN CONTACT WITH A MOLTEN SALT WHEREIN THE CATION THEREOF IS SELECTED FROM THE GROUP CONSISTING OF LITHIUM, SODIUM, POTASSIUM, CALCIUM, BARIUM, CADMIUM, LEAD, ANTIMONY, AND SODIUM ALUMINUM; AND WHEREIN THE ANION IS SELECTED FROM THE GROUP CONSISTING OF SULFATE, CHLORIDE, FLUORIDE, SULFIDE, AND CYANIDE, FOR A CONTACT TIME OF 0.005-0.1 SECOND, SEPARATING THE MOLTEN SALT FROM THE CONVERSION PRODUCT, SAID PRODUCT COMPRISING SUBSTANTIAL REDUCTION IN NON-AROMATIC HYDROCARBONS IN THE NORMALLY LIQUID BOILING RANGE AND SUBSTANTIAL FORMATION OF NORMALLY GASEOUS HYDROCARBONS. 